I noticed a HUGE battery drain with other VPN clients like PIA's app or just OpenTunnel, but either my 3 year old iPhone X has a great battery, or the battery drain from WireGuard has been unnoticeable for me.
I use an iPhone SE for work with a WireGuard VPN, no noticeable drain (beyond that battery life is generally pretty bad compared to my Android, even when WiFi/data/bluetooth/gps are turned off).